The present invention relates to a driving force distributing apparatus for a four-wheel drive vehicle, in which most of a driving force output of an engine is directly transmitted to main drive wheels, and a required minimum output thereof is transmitted to auxiliary drive wheels.
In the prior art, known is a technique for controlling a distribution of the driving force such that the power is transmitted directly to either one of a front-wheel drive system and a rear-wheel drive system and the power is transmitted through a coupling unit such as a clutch to the other drive system.
In this kind of the four-wheel drive vehicles, there have been many trial experiments for lowering necessary strength of such a power transmission mechanism as the coupling unit and an axle as a differential for the four-wheel drive by lowering a transmitting capacity to the auxiliary drive wheels as much as possible for making the four-wheel drive mechanism light in weight. However when starting on an uphill road with a low-friction coefficient xcexc, an excessive slip of the main drive wheels is liable to occur to raise a problem that a sufficient traction performance cannot be achieved as the four-wheel drive.
In Japanese Patent Unexamined Publication No. 9-240301, therefore, there has been disclosed a following technique. In the technique, front wheels as the main drive wheels and a hydraulic pump are driven by the engine, and the power drives the vehicle with the front wheels. On the other hand, oil pumped by a hydraulic pump is fed to a variable displacement pump motor. Then, rear wheels as an auxiliary drive wheels are driven by a variable displacement pump motor. If a slippage occurs at a start on the low-xcexc road such as a icy road or a road covered with snow, a number of rotations of the front wheels exceeds that of the rear wheels. According to the difference of the numbers between those of the front and rear two wheels, therefore, the transmitted torque of the variable displacement pump motor to the rear wheels is set variable. At the same time, in the case of running on a steep uphill, a high driving torque is transmitted to the rear wheels on the basis of the output value detailed by an inclinometer.
According to the technique thus disclosed in that Application, the torque corresponding to the slippage amount can be transmitted to the auxiliary drive wheels so that the torque to be transmitted to the auxiliary drive wheels can be reduced to the necessary minimum value.
According to the technique disclosed above, however, the torque in the auxiliary drive wheels do not increase before the slippage occurs in the main drive wheels, when the torque to be transmitted to the auxiliary drive wheels is detected in terms of the revolution difference of the main drive wheels due to the slippage. For the vehicle aiming at increasing a climbing ability for a road gradient of 20 degrees with the road friction coefficient of 0.03, for example, the torque to be transmitted to the auxiliary drive wheels will increase by 40% or more than that of no slippage, if it is assumed that the slippage occurs in the main drive wheels so that the friction coefficient between the road surface and the wheels drops to 0.1. There is required further torque for preventing the slippage in the main drive wheels.
The torques have to be increased by about 20% or more, although the torques are determined on the moment of inertia in the main drive wheel system and on how the slippage was avoided for a short time period.
When the slippage occurs in the main drive wheels, more specifically, the capacity of the torque to be transmitted to the auxiliary drive wheels has to be required 1.6 times or more than that of the target performance torque. This causes a defect that the entire apparatus becomes large and heavy.
In order to detect the road gradient, moreover, a unique inclinometer has to be specially provided. Because of this, it is raised a problem that the number of parts and manufacturing cost are increased.
In view of the background thus far described, it is an object of the present invention to provide a driving force distributing apparatus for a vehicle, which can prevent the slippage of the main drive wheels reliably at the slope running time including the start, without increasing the torque to be transmitted to the auxiliary drive wheels, more than necessary, to thereby reduce the size of the entire system, and also minimize the rise in the cost without increase the number of parts.
In order to achieve the above-specified object, according to a first aspect of the invention, there is provided a driving force distributing apparatus for a vehicle (e.g., a four-wheel drive vehicle), for transmitting a driving power directly to main drive wheels, and to auxiliary drive wheels via a coupling unit. The apparatus comprises basic torque setting means for setting a basic torque to be transmitted to the auxiliary drive wheels on the basis of driving states of the vehicle, road gradient correction coefficient setting means for determining a gradient correction coefficient corresponding to a gradient of a road by comparing an output value of a longitudinal acceleration sensor mounted on the vehicle and an acceleration determined on the basis of a speed of the vehicle, auxiliary drive wheel torque setting means for setting an auxiliary drive wheel torque by correcting the basic torque with the gradient correction coefficient and driving means for driving said coupling unit to generate a transmission torque corresponding to the auxiliary drive wheel torque.
With this construction, first of all, a basic torque to be transmitted to the auxiliary drive wheels connected to the prime mover via the transmission torque capacity variable type coupling unit is set on the basis of a driving state of a vehicle. A gradient correction coefficient corresponding to the gradient of a road is determined by comparing an output value of a longitudinal acceleration sensor mounted on the vehicle and an acceleration determined on the basis of a vehicle speed. An auxiliary drive wheel torque is set by correcting the basic torque with the gradient correction coefficient. The transmission torque capacity variable type coupling unit is caused by drive means to generate a transmission torque corresponding to the auxiliary drive wheel torque.
According to a second aspect of the invention, there is provided a driving force distributing apparatus for a vehicle (e.g., a four-wheel drive vehicle) for transmitting an engine output directly to main drive wheels, and to auxiliary drive wheels via a coupling unit. The apparatus comprises auxiliary drive wheel torque setting means for setting an auxiliary drive wheel torque on the basis of a road gradient determined on the basis of an output value of a longitudinal acceleration sensor mounted on the running vehicle at a low speed or at a stop and driving means for driving said coupling unit to generate a transmission torque corresponding to the auxiliary drive wheel torque.
With this construction, first of all, a road gradient is determined on the basis of an output value of a longitudinal acceleration sensor mounted on a vehicle at a driving time of an extremely low speed or at a stop. Then, an auxiliary drive wheel torque is set on the basis of the road gradient and coupling unit which has the variable transmission torque capacity is caused to generate a transmission torque corresponding to the auxiliary drive wheel torque.
According to a third aspect of the invention, there is provided a driving force distributing apparatus for a vehicle (e.g., a four-wheel drive vehicle) for transmitting an engine output to main drive wheels, and to auxiliary drive wheels via a coupling unit. The apparatus comprises basic torque setting means for setting a basic torque to be transmitted to the auxiliary drive wheels, on the basis of a driving state of the vehicle, theoretical acceleration setting means for setting a theoretical acceleration at a time of running on a flat road on the basis of an equation of motion of the vehicle from the engine output and a driving force determined by preset vehicle specifications, estimated acceleration setting means for computing an estimated acceleration from a wheel speed, auxiliary drive wheel torque setting means for setting an auxiliary drive wheel torque on the basis of a road gradient determined by comparing said theoretical acceleration and the estimated acceleration and driving means for driving said coupling unit to generate a transmission torque corresponding to the auxiliary drive wheel torque.
With this construction, first of all, a basic torque to be transmitted to the auxiliary drive wheels is set on the basis of a driving state of a vehicle, a theoretical acceleration at the time of running on a flat road is set on the basis of the equation of motion of the vehicle from the output of the prime mover and the driving force determined by preset vehicle items. An estimated acceleration is computed from a wheel speed. An auxiliary drive wheel torque is set on the basis of a road gradient determined by comparing a vehicle body acceleration and the estimated acceleration and drive means causes the transmission torque capacity variable type coupling unit for connecting the prime mover and the auxiliary drive wheels to generate a transmission torque corresponding to the auxiliary drive wheel torque.
In this case, in the present invention it is preferable that the auxiliary drive wheel torque setting means sets a corrected auxiliary drive wheel torque by correcting the set auxiliary drive wheel torque with a steering angle correction coefficient set on the basis of a steering angle.
Further, in the present invention, it is advantageous that the auxiliary drive wheel torque set by the auxiliary drive wheel torque setting means is reduced and corrected after the start.
Furthermore, in the present invention, it is advantageous that the corrected auxiliary drive wheel torque set by the auxiliary drive wheel torque setting means is reduced and corrected after the start.
Moreover, in the present invention, it is advantageous that the steering angle correction coefficient has characteristics to decrease in proportion to the steering angle.